1. Field of the Invention
The present invention relates generally to semiconductor circuit processing, and more specifically to a method of field isolation using spin-on-glass (SOG) polymers.
2. Description of the Related Art
Local oxidization of silicon (LOCOS) is the conventional lateral isolation scheme. Referring to FIG. 1, in the LOCOS technique, a layer of silicon nitride 14 is deposited over a barrier oxide 12 overlying a silicon substrate 10. The barrier oxide 12 is a thin oxide which allows better adhesion between the nitride and silicon and acts as a stress relaxation layer during field oxidation. The nitride and oxide layers 14,12 are etched to leave openings 11 exposing portions of the silicon substrate where the local oxidation will take place. A boron channel-stop layer is ion implanted into the isolation regions. The field oxide 15 is grown within the openings 11 and the nitride and barrier oxide layers 14, 12 are removed. This completes the local oxidation.
When the silicon nitride openings 11 between active device regions are reduced to the sub-micrometer regime, a reduction in field oxide thickness within the openings is observed. This oxide thinning in the area where field spacing is narrow causes insufficient isolation between active regions 13. Thus, the scaling capability of the LOCOS technique is severely restricted by lateral encroachment of the oxide (often referred to as bird's beak) into the active region 13. As shown in FIG. 1, in LOCOS isolation, scalability is limited by oxide encroachment under the nitride during field oxidation. As this encroachment can typically be as high as 80 percent of the field oxide thickness [Lbb/Tfox=0.8], at spacings of 1.0 um, and a field oxide thickness of 5000 A, the bird's beak encroachment may be as high as 0.8 um, leaving only 0.2 um of active region 13.
Several enhancements to conventional LOCOS involve either additional mask levels and complex processing sequences or employ processes, such as lift-off, whose control and reliability are questionable.
One such technique is known as Side Wall Masked Isolation (SWAMI) described by K. Y. Chiu et al in IEDM Tech. Digest, pp 224-227, December 1982 and in IEEE Trans. Electron Devices, vol. ED-29, pp 536-540, 1982. In this technique, the silicon substrate is trenched before the oxidation step and thus stress, which often leads to defects in the silicon substrate, is a problem.
Another such technique is known as poly-buffered LOCOS which is described by R. L. Guldi et al. in J. Electrochem. Soc., 136, 3815, 1989; N. Hoshi et al., Proceedings of IEDM Meeting, p. 300, December 1986; and R. A. Chapman et al., Proceedings of IEDM Meeting, p. 362, December 1987. In this technique, in addition to the complexity of process steps, the danger exists of a double bird's beak if the polysilicon is allowed to be oxidized before the nitride deposition.
Yet another known technique uses a spin-on-glass polymer. A spin-on-glass (SOG) composition is a liquid, silica-based composition that can be applied to the surface of a semiconductor wafer and spun with the wafer to provide a coating with a level top surface. As reported by G. Smolinsky, et al. in J. Electrochem. Soc., 137, 229, 1990, in this technique, SOG is filled in the trenches formed in the substrate, two different masks are used to accommodate large field areas and the SOG is annealed in nitrogen and dry oxygen for 30 minutes at 900.degree. C. SOG is unstable and with this technique, the properties of the SOG are altered only slightly without approaching the properties of a thermal oxide, as reported by M. P. Woo, J. L. Cain and C. O. Lee in J. Electrochem. Soc, 137, 196 (1990) and by E. E. Ibok and S. Garg in Abstract No. 862, Proceedings of the 183rd Meeting of the Electrochem. Soc., Honolulu, Hi. (1993). Moreover, the different thermal expansion of the spin-on-glass and the underlying substrate contributes to the cracking of the spin-on-glass, thereby yielding poor reliability.
Furthermore, when wide isolation fields are required, a dip is formed in the spin-on-glass surface and the spin-on-glass does not retain a planar shape. A known solution is to fill up the isolation trenches with multiple spins. However, the additional spin-on-glass spins also add the spin-on-glass polymer on the active regions of the substrate, thereby requiring additional processing to remove the spin-on-glass polymer from the active regions. This additional processing is expensive and thus undesirable.